JSON Pagination (20 data per page)

Question

I want to get the first 20 data for the first page from the url below. I want to get the next 20 data on the second page. I don't want to get all the data in the first time. I just want to get the first 20 data.

URL:

https://rss.itunes.apple.com/api/v1/us/books/top-free/all/%20100/non-explicit.json

JSON:

class BookProvider {

    // MARK: - Properties
    private let baseUrl = "https://rss.itunes.apple.com/api/v1"

    // MARK: - Life Cycle
    init() { }

    // MARK: Functions
    func loadTopCharts() -> Future<[Book], Error> {
        let urlString = "us/books/top-free/all/100/non-explicit.json"

        if let data = getData(from: urlString) {
            if let songs = jsonDictionary(from: data)?["results"] {
                do {
                    let data = try JSONSerialization.data(withJSONObject: songs, options: .fragmentsAllowed)
                    let bookDecoder = try JSONDecoder().decode([Book].self, from: data)

                    return Future { promixe in
                        promixe(.success(bookDecoder))
                    }
                }catch {
                    debugPrint(error.localizedDescription)
                }
            }
        }
        return Future{ _ in }
    }

    private func getData(from urlString: String) -> Data? {
        guard let url = URL(string: baseUrl + "/" + urlString) else {
            fatalError("Couldn't load URL from provided string.")
        }

        do {
            return try Data(contentsOf: url)
        } catch {
            debugPrint("Couldn't load data from URL:\n\(error)")
            return nil
        }
    }

    private func jsonDictionary(from data: Data) -> [String : Any]? {
        do {
            let dictionary = try JSONSerialization.jsonObject(with: data, options: .allowFragments)
            return (dictionary as? [String : Any])?["feed"] as? [String : Any]
        }catch {
            debugPrint(error.localizedDescription)
            return nil
        }
    }
}